1. Field of the Invention
The present invention relates to a developing device used in an image forming apparatus of an electrophotographic or electrostatic recording scheme to develop an electrostatic image on an image holding member.
2. Description of the Related Art
In an image forming apparatus of the electrophotographic scheme, an electrostatic image formed on the photosensitive member is developed with toner in the developing device.
In such a developing device, a developer is held by the developing sleeve placed so as to oppose the photosensitive member in order to perform developing. So, toner must be prevented from leaking out via the end portions of the developing sleeve.
To do this, seal members are provided for the developing sleeve end portions. As such a seal member for preventing leakage of toner, an elastic member, such as a felt or foam rubber member has widely been used. FIGS. 9 and 10 show a typical example of this member. FIG. 9 is a sectional front view showing the main part of a developing device. FIG. 10 is a sectional side view showing the main part of the developing device.
As shown in FIG. 9, a developing sleeve 5 incorporates a magnetic roller 6. The developing sleeve 5 is rotatably supported on a developer container 3 through a sleeve bearing 12, as shown in FIG. 10. With this arrangement, toner supplied from the developer container 3 adheres to the surface of the developing sleeve 5 owing to the magnetic force of the magnetic roller 6, and is regulated by a developing blade 7 to a predetermined thickness. Then, with rotation of the developing sleeve 5, the toner adheres to a latent image on the photosensitive drum 1 at a position opposite thereto, thus developing the image. Elastic seal members 8 are mounted on the two longitudinal end portions of the developing sleeve 5 outside the developing area. More specifically, the elastic seal members 8 are mounted on the front portion (on the opening side) of the developing sleeve 5, which is mounted on the developer container 3, and the rear portion (on the opposite side to the opening side) of the developing sleeve 5. These elastic seal members 8 are pressed against the outer surface of the developing sleeve 5 to prevent leakage of the toner.
In this sealing method of pressing the elastic members against the developing sleeve, the torque required to drive the developing sleeve during a developing operation becomes large.
In addition, as the number of times the elastic seal members are used increases, the sealing ability of each seal member deteriorates.
The sealing method of pressing the elastic members against the developing sleeve is not suitable for a developing device to realize a higher operation speed and a longer service life.
Under the circumstances, a technique of forming magnetic seals has been proposed. According to this technique, magnets are arranged at the two ends of a developing sleeve through a gap, and magnetic seals are formed by the magnetic fields generated by the magnets outside the developing sleeve and the magnet inside the developing sleeve.
FIG. 11 is a sectional front view showing an example of a developing device using magnetic seal members. Referring to FIG. 11, magnetic seal members 20, made of magnets, are wound around the two end portions of a developing sleeve 5 to oppose its outer surface through a predetermined gap g. In this state, the magnetic seal members 20 are mounted on a developer container 3, together with the developing sleeve 5. Each magnetic seal member 20 is magnetized to a magnetic pole pattern like the one shown in FIG. 12. The gap g between the outer surface of the developing sleeve 5 and the surface of each magnetic seal member 20 is filled with a magnetic brush due to a triboelectric brush of the toner formed along magnetic lines of force 24, thereby preventing the toner from leaking out of the developing area.
As a magnetic seal member, a member having the magnetization pattern shown in FIG. 13 or 14 may be used.
With the use of this technical means, since the developing sleeve 5 and the magnetic seal members 20 can be arranged in a non-contact state, the rotational torque of the developing sleeve 5 is considerably small. For this reason, a compact, inexpensive driving motor can be used. In addition, since variations in rotational torque are small, the developing sleeve 5 and the photosensitive drum 1 do not easily undergo rotation variations. This technique is therefore suitably used to increase the operation speed of the device.
Furthermore, since the magnetic seal members 20 are free from wear and the like, they can be used semipermanently and recycled.
In this magnetic sealing method, the sealing performance is influenced by the magnetic flux density.
Although the sealing effect can be enhanced by using magnets having strong magnetic forces, the device increases in size, and the magnetic forces may inflict adverse effects on the developing area.